The invention pertains to devices which allow converting a displacement into a force and vice-versa.
In known devices, such a conversion is provided by a spring, generally a helical spring whose pitch gradually decreases toward the ends, which are ground to provide flat support surfaces.
This type of spring does not yield a linear conversion of a displacement into a force. In fact, considering the expression for the rigidity of a helical spring: R.sub.c =Gd.sup.4 /8D.sup.3 n, where G is the shear modulus, d is the wire diameter, D is the average diameter of the spring and n is the number of loops, it is seen that, for this type of spring whose pitch decreases gradually toward the ends and whose number of loops varies directly with the compression, the rigidity varies and leads to an error in the conversion of the motion into a force. Examples of prior spring devices known to applicants are shown in the following patents: French Pat. Nos. 995,091 and 1,288,773; U.S. Pat. Nos. 2,695,169 and 3,951,391; British Pat. No. 260,707 of 1926, 192,578 of 1923 and 1,237,071 of 1971; Swiss Pat. No. 236,279; and German Pat. No. 1,282,361. The above patents show grooved end caps for helical springs, flat ends or other means engaging certain end loops but none of them solves the problem recognized by the present applicants, as fully described hereafter.